Multi-channel signal processing in an optical reader

ABSTRACT

An optical reader, for example a field of view reader or a flying spot scanner for reading a printed indicia such as a bar code symbol includes a light source and a light detector, and at least two channels associated with the detector for carrying signals corresponding to light detected by the detector at different resolution levels, thus simplifying the decoupling of signals allowing a single reader to be used regardless of the resolution level of the indicia to be read. 
     In order to arrive at an improved signal to noise ratio of a bar code symbol being read, a processor produces two signals at respective first and second channels, the signal in the second channel being buffered and merged with a later signal in the first channel allowing time averaging out of the noise portion of the signal and enhancement of the information portion of the signal. 
     An improved system for detecting defects in a printed bar code symbol includes processing means for comparing the width of a detected space with the width of neighboring spaces and identifying the space as a defect if its width is less than a predetermined proportion of the width of neighboring spaces.

REFERENCE TO RELATED APPLICATIONS

This application is a divisional of Ser. No. 09/759,937, filed Jan. 12,2001, now U.S. Pat. No. 6,435,412 which is a divisional of Ser. No.08/598,928, filed Feb. 9, 1996, now U.S. Pat. No. 6,213,399, which is acontinuation-in-part of Ser. No. 08/314,519, filed Sep. 28, 1994, nowU.S. Pat. No. 5,506,392, which is a divisional of Ser. No. 08/109,021,filed Aug. 19, 1993, now U.S. Pat. No. 5,352,922, which is a divisionalof Ser. No. 07/735,573, filed Jul. 25, 1991, now U.S. Pat. No.5,278,397.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an optical reader including a multi-channeldetector.

Various readers and optical scanning systems have been developed forreading printed indicia such as bar code symbols appearing on a label orthe surface of an article and providing information concerning thearticle such as the price or nature of the article. The bar code symbolitself is a coded pattern of indicia comprised of, for example, a seriesof bars of various widths spaced apart from one another to form spacesof various widths, the bars and spaces having different light reflectingcharacteristics. The readers electro-optically transform the graphicindicia into electrical signals which are decoded into alpha-numericcharacters that are intended to be descriptive of the article or acharacteristic thereof. Such characters typically are represented indigital form, and utilised as an input to a data processing system forapplications in point of sale processing, inventory control and thelike.

Known scanning systems comprise a light source for generating a lightbeam incident on a bar code symbol and a light receiver for receivingthe reflected light and decoding the information contained in the barcode symbol accordingly. The readers may comprise a flying spot scanningsystem wherein the light beam is scanned rapidly view reading systemwherein the bar code symbol to be read is illuminated as a whole and aCCD (Charge Coupled Device) array is provided for detecting the lightreflected from the bar code symbol. The reader may be either a hand-helddevice or a surface-mounted fixed terminal.

A variety of scanning devices are known. The scanner could be a wandtype reader including an emitter and a detector fixedly mounted in thewand, in which case the user manually moves the wand across the symbol.Alternatively, an optical scanner scans a light beam such as a laserbeam across the symbol, and a detector senses the light reflected fromthe symbol. Alternatively a gun-type hand-held arrangement may beprovided. In either case, the detector senses reflected light from aspot scanned across the symbol, and the detector provides the analogscan signal representing the encoded information.

A digitizer processes the analog signal to produce a pulse signal wherethe widths and spacings between the pulses correspond to the widths ofthe bars and the spacings between the bars. The digitizer serves as anedge detector or wave shaper circuit, and the threshold value set by thedigitizer determines what points of the analog signal represent baredges. The threshold level effectively defines what portions of a signalthe reader will recognize as a bar or a space.

Readers of the type discussed above are single channel systems having asingle digitizer output and/or a single processing chain to produce asingle digitized output.

The pulse signal from the digitizer is applied to a decoder. The decoderfirst determines the pulse widths and spacings of the signal from thedigitizer. The decoder then analyses the widths and spacings to find anddecode a legitimate bar code message. This includes analysis torecognize legitimate characters and sequences, as defined by theappropriate code standard. This may also include an initial recognitionof the particular standard the scanned symbol conforms to. Therecognition of the standard is typically referred to as autodiscrimination.

Different bar codes have different information densities and contain adifferent number of elements in a given area representing differentamounts of encoded data. The denser the code, the smaller the elementsand spacings. Printing of the denser symbols on a appropriate medium isexacting and thus is more expensive than printing low resolutionsymbols.

A bar code reader typically will have a specified resolution, oftenexpressed by the size of its effective sensing spot. The resolution ofthe reader is established by parameters of the emitter or the detector,by lenses or apertures associated with either the emitter or thedetector, by the threshold level of the digitizer, by programming in thedecoder, or by a combination of two or more of these elements.

In a laser beam scanner the effective sensing spot may correspond to thesize of the beam at the point it impinges on the bar code. In a scannerusing an LED or the like, the spot size can be the illuminated area, orthe spot size can be that portion of the illuminated area from which thedetector effectively senses light reflections. By what ever means thespot size is set for a particular reader, the photodetector willeffectively average the light detected over the area of the sensingspot.

A high resolution reader has a small spot size and can decode highdensity symbols. The high resolution reader, however, may have troubleaccurately reading low density symbols because of the lower qualityprinting used for such symbols. This is particularly true of dot matrixtype printed symbols. The high resolution reader may actually sense dotwidths within a bar as individual bar elements. In contrast, a lowresolution reader has a large spot size and can decode low densitysymbols. However, a reader for relatively noisy symbols such as dotmatrix symbols reads such a wide spot that two or more fine bars of ahigh resolution symbol may be within the spot at the same time.Consequently, a reader having a low resolution, compatible with dotmatrix symbols can not accurately read high density symbols. Thus anyreader having a fixed resolution will be capable of reading bar codesonly within a limited range of symbol densities.

For a given symbol density, the resolution of the reader also limits therange of the working angle, i.e. the angle between the axis of thereader and a line normal to of the surface on which the bar code isprinted. If the range and resolution are too limited, a user may havedifficulty holding a hand-held reader comfortably while accuratelyscanning the bar code. This can be particularly troublesome if thereader incorporates additional elements to form an integrated dataterminal. The combination of size, weight and an uncomfortable angle canmake reading in large amounts of bar code information difficult andannoying, and thereby make the user more resistant to use of the barcode system. Problems also arise with fixed readers which may have to bemanually switched dependent on the resolution required, reducingefficiency and slowing operation of the system.

One solution might be to provide some means to adjust the resolution orsensing spot size of the reader, e.g., by adjusting the threshold of thedigitizer. This approach, however, would require a number of differentscans at different resolutions. If the scan is automatic, the variationin resolution causes a loss of robustness because the scan is at thecorrect resolution only a reduced amount of the time. Effectively such ascanner would scan at the equivalent of a reduced rate. If the reader isa hand-held device, the user would have to manually scan the readeracross the information each time the resolution changes. This causes amarked reduction in the first read rate and increased frustration forthe user.

In addition most optical scanners such as bar code scanners are adaptedfor use at a particular distance, or a range of distances, from anindicia to be scanned. If the user holds the scanner too close to theindicia, (or, conversely the object is held too close to the scanner) ortoo far away, the indicia and/or the flying spot beam will not be infocus, and decoded will not be possible.

Such scanners may not be particularly convenient in environments where aseries of indicia to be read are presented to the scanner at variousdistances, and where it is difficult or impossible for the user to alterthe distance between the scanner and the indicia. To deal with suchsituations, attempts have been made to expand the acceptable workingrange of conventional scanners, to give the user as much leeway aspossible, and also to provide multi-distance scanners which can operate,for example, at a first working range or at a second working rangeaccording to the user's preference or requirements. One possibility isfor the provision of a two-position switch on the scanner, with thescanner operating at a first working distance in a first position of theswitch and at a second working distance in a second position.

An improved solution is to provide an optical scanner including amulti-surface reflector having a first surface of a first profile and asecond surface of a second profile, the scanner being adapted to read anindicia at a first distance or working range when the beam is reflectedfrom the first surface and at a second distance or working range whenthe beam is reflected from the second surface.

A further problem associated with conventional methods of signalprocessing is that the analog signal is processed only once and thedecodeability of the processed signal is principally determined by thesignal to noise ratio of the incoming signal.

In addition known digitizers have been recognised as requiringimprovement in the scanning of dot matrix symbols such as bar codesymbols printed in dot matrix format. The failure to decode such symbolsis commonly caused by a split in a wide bar wherein a void in the barcauses a narrow element to appear in the middle of the bar. This onlyhappens in the case of wide bars because such bars are built up of twoor three columns of dots and small spaces between the columns aresometimes large enough to be recognised and digitized as wide elements.It will be appreciated that the problem does not arise with narrow barsmade up a single column of dots.

SUMMARY OF THE INVENTION

Objects of the Invention

It is an object of the present invention at least to alleviate theproblems of the prior art. It is a further object to provide a readerallowing improved reading of printed indicia at multiple resolutions.

It is a further object still of the present invention to provideimproved reading of printed indicia presented at various distances fromthe reader.

It is still a further object of the invention to provide an opticalreader having an improved signal to noise ratio.

It is yet a further object of the invention to provide further improvedreading of dot matrix symbols such as bar code symbols.

Features of the Present Invention

According to the invention there is provided an optical reader forgenerating an outgoing light beam to illuminate an information symbolcomprising regions of different light reflectivity, and for collectingan incoming light beam reflected from the symbol, the reader comprising:

a source of the outgoing light beam;

light collection optics for collecting the incoming light beam and fordirecting the incoming light beam to a multi-channel photodetector;

the multi-channel photodetector comprising an array of more than oneindividual active optical sensing element, each said element having anoutput capable of providing an output signal representative of lightimpinging thereon;

the output of each detection element being coupled to a respectivechannel for processing the output signal of a respective detectionelement. Because a plurality of channels are involved, improved couplingout of signals at each channel is achieved. This is of particularadvantage when it is desired to convey different signal information ineach channel.

One of the active optical sensing elements may be positioned aroundanother of the active optical sensing elements. Preferably the outgoinglight beam, light collection optics, and array of sensing elements areconfigured so that the output of a first group of the sensing elementscorresponds to a spot of a first size passing across the informationsymbol, and so that the output of a second group of the sensing elementscorresponds to a spot of second size, larger than said first size,passing across the information symbol. Accordingly each channel willtransfer a signal at a different resolution level.

The array of sensing elements may comprise a first and a second sensingelement, and the first sensing element may be contained within the firstgroup and the first and the second elements are contained within thesecond group. The first group may contain only the first sensingelement, and the second group may contain only the first and secondsensing elements.

According to the invention there is further provided a reader forreading printed indicia, for example bar code symbols, comprising alight source for illuminating an indicia and a light detector forproducing a signal corresponding to detected light reflected from theindicia, the light detector having a plurality of channels, each channelbeing associated with a different resolution level of the signalproduced by the detector. Thus symbols or indicia at differentresolutions can be read quickly and simply by a single reader.

The detector may be arranged to process the signal and transfer a signalprocessed at a first threshold level via a first channel and a signalprocessed at a second resolution level via a second channel.

Alternatively the light source may be arranged to generate a firstilluminating beam at a first resolution and a second illuminating beamat a second resolution, the first and second illuminating beams beingspatially separated, and the detector may comprise a first regionassociated with a first channel for producing a signal corresponding tothe first beam and a second region associated with a second channel forproducing a signal corresponding to the second beam. The light sourcemay be arranged to generate a first and second illuminating beam spacedlaterally from one another and the detecting regions of the detectorcorresponding to the first and second beams may be laterally spaced fromone another. Alternatively the light source is arranged to generateconcentric first and second illuminating beams and the detecting regionsof the detector corresponding to the first and second beams may beconcentric.

The light source may be arranged to generate a light beam of a firstresolution at a first wavelength and a light beam of a second resolutionat a second wavelength, and the detector may be arranged to detect lightof the first wavelength and produce a signal corresponding to the firstlight beam at a first channel and to detect light of the secondwavelength and produce a signal corresponding to the light of the secondwavelength at a second channel.

The light source may be arranged to generate a light beam at a firstresolution in a first set of pulses and a light beam at a secondresolution in a second set of pulses temporally off-set from the firstset of pulses, and the detector may be synchronised with the lightsource to detect light pulses of the first resolution and producecorresponding signals at a first channel and to detect the light pulsesof the second resolution and produce corresponding signals at a secondchannel.

The reader may comprise a flying spot scanning reader or a field of viewoptical reader. A decoder may be arranged to analyse the signal from achannel to determine whether it is a valid signal and arranged to mergethe signals from each channel if no valid signal from an individualchannel is detected.

According to the invention there is further provided a reader forreading printed indicia comprising a light source for illuminating anindicia and a light detector for detecting light reflected from theindicia wherein the detector comprises a first region and a secondregion for detecting reflected light at a first resolution level and asecond resolution level respectively, a first channel being associatedwith the first region and a second channel being associated with thesecond region.

According to the invention there is further provided a method ofoperation of a reader for reading printed indicia comprising a lightsource for illuminating the indicia and a light detector producing asignal corresponding to detected light reflected from the indicia, afirst channel and a second channel being associated with the detectorfor carrying respective signals at different resolution levels, and adecoder; the method including the steps of decoding the signal from thefirst channel and, if that signal is not valid, decoding the signal fromthe second channel and, if that is not valid, merging the signals fromthe first and second channels, decoding the merged signal and selectingthe valid portions of the merged signal.

According to the invention there is further provided a reader forreading printed indicia such as bar code symbols comprising a lightsource for illuminating an indicia and a detector for producing a signalcorresponding to detected light reflected from the indicia, a firstchannel and a second channel being arranged to receive signals processedby the detector wherein the second channel includes buffer means fordelaying a signal and merging the signal with a subsequent signaltransferred by the first channel. As a result, the signal portion isenhanced and the noise portion reduced giving rise to an improved signalto noise ratio.

An indicia may be scanned at intervals T giving rise to a signal to thefirst and second channels via the detector, and the buffer means of thesecond channel may be arranged to delay the signal for the period T inorder to merge the signal with the subsequent signal transferred by thefirst channel.

According to the invention there is further provided a method ofprocessing analog signals corresponding to a light beam reflected from aprinted indicia including receiving the light beam at a detector,converting the light beam to a digital signal and transferring thedigital signal via a first channel to a decoder, transferring thedigital signal from the detector via a second channel to a delay bufferand merging the delayed signal with a subsequent signal transferred bythe first channel.

According to the invention there is further provided a method ofdecoding a bar code symbol composed of elements of differentreflectivity comprising scanning the bar code symbol with a light beam,detecting the reflected light beam and providing a digitized signalcarrying the bar code information, wherein the decoder compares thewidth of each element of a first reflectivity with the width of anotherelement of that reflectivity in the bar code symbol and identifies theelement as a defect if its width is less than a predetermined proportionof the width of the other element. As result, defects in a printed barcode symbol, for example because of defectively printed dark regions,may be detected.

The predetermined proportion may be 60%.

If the element is identified as a defect its width may be added to thewidth of the elements on either side of the element. It is thus possibleto determine the width, for example, of a bar, if the intermediateelement is a space.

The element may be compared with neighbouring elements of the samereflectivity.

According to the invention there is further provided a processor forprocessing an optical signal received from a printed indicia, theprocessor including a detector, a first channel and a second channelassociated with a detector and a decoder wherein the channels arearranged to transfer signals at different resolution levels from thedetector to the decoder.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing objects and advantages of the present invention may bemore readily understood by one skilled in the art with reference beinghad to the following detailed description of several preferredembodiments thereof, taken in conjunction with the accompanying drawingswherein like elements are designated by identical reference numeralsthroughout several views, and in which:

FIG. 1 is a perspective view of a conventional scanner;

FIG. 2 a illustrates scanning of sensing spots across a bar code symbol;

FIG. 2 b illustrates scanning of sensing spots across a dot matrixsymbol;

FIG. 3 is a block circuit diagram of an embodiment of the invention;

FIG. 4 is a block circuit diagram of an alternative embodiment of theinvention;

FIG. 5 is a block circuit diagram of an alternative embodiment of theinvention;

FIG. 6 is a simplified plan view of a detector having two active areas;

FIG. 7 is a more detailed view of the detector of FIG. 6;

FIG. 8 is a more detailed view of an alternative detector of FIG. 6;

FIG. 9 is a block circuit diagram for the invention including thedetector of FIG. 6;

FIG. 10 is a block diagram of a circuit according to another aspect ofthe invention;

FIG. 11 illustrates operation of the arrangement shown in FIG. 10; and

FIG. 12 is a flow chart illustrating operation of a system according toa further aspect of the invention.

FIG. 1 illustrates a typical bar code reader. The reader comprises ahand-held laser scanner, generally indicated at 700, having a main body535 including a graspable hand portion 536 which carries a trigger 539.Within the body 535 is a laser module 515 (not shown in detail). Lightfrom the laser module 515 is arranged to shine onto an oscillatingmirror provided in the body 535. The resultant reflected beam passesthrough a lens, and out of the housing via a window 538. The mirror isarranged to oscillate in such a way that the beam traces out a scan line513 across a bar code 600 to be recorded. In the example shown in FIG.1, the bar code 600 is a linear bar code, but it will of course beappreciated that the scanner could instead be arranged to readtwo-dimensional bar codes: in such a case the mirror would be controlledto produce a raster scan rather than a single line scan.

Light reflected back from the bar code 600 passes through the window538, is collected by a collecting mirror, from where it is reflectedback to a photodetector. The optical signal is then converted into anelectrical signal, and the features of the bar code symbol 600determined.

The arrangements discussed below may be included in such a reader or inany other type of reader, for example a laser wand reader, a fixedreader and either flying spot or field of view readers.

The print quality and print density of indicia to be read, for examplebar code symbols, varies widely. For example, low resolution bar codesymbols may comprise relatively wide bars printed in dot matrix formatwhereas high density symbols will comprise bars or elements that arerelatively small in dimension. Accordingly a bar code reader thatgenerates a scanning spot that is relatively broad and thus suitable forreading low density bar code symbols may not be appropriate for readinghigh density symbols as the broad spot will encompass more than oneelement and group them together as a single element. Conversely, a barcode reader that generates a very narrow spot suitable for reading highdensity indicia may not be suitable for low density indicia printed indot matrix format as the dimensions of the small spot may be similar tothe dimensions of the individual dots and spaces in the dot matrixformat as a result of which a single bar comprising a plurality ofcolumns of dots may be read as a large number of narrow elements. Theproblem is exacerbated further if the reading beam is incident on thesymbol at an angle to the normal as the reading spot will becorrespondingly spread out leading to further problems with resolution.

To illustrate the situation, FIG. 2 a shows scanning of the spots S₁ andS₂ across a bar code 20, for a relatively small working angle 0°. Theuser keeps the working angle close to or equal to 0° throughout thelength of the scan, and the spots S₁ and S₂ both remain essentiallycircular.

Scanning of the spots S₁ and S₂ is shown across a relatively highdensity bar code 20. At several points of the scan of the high densitybar code, particularly at the middle two positions illustrated in thedrawing, the larger sensing spot S₁ covers two or more elements. Theaveraging over such an area would not provide an accurate signalindicating the edges of the small bar code elements. The smallerdiameter spot S₂, however, covers so small an area that even at thesepositions it covers only one narrow element and thus provides anaccurate reading.

FIG. 2 b illustrates scanning of the spots S₁ and S₂ over one element 23of a dot matrix bar code. As shown, the element 23 actually exiutitsgaps between the individual dots. Sensing using the small spot S₂ willdetect the dots as dark areas and the taps as light regions. Forexample, at the position shown in FIG. 2 b, the spot S₂ coincidesapproximately with one of the gaps. As a result, the signal responsiveto sensing of spot S₂ would, at that point indicate a light space, not adark bar element, Thus sensing of the spot S₂ would not accurately readthe width of the element 23. The larger spot S₁, however, should producea legitimate decode result. Sensing wing spot S₁ averages the reflectedlight over the larger area of that spot and consequently would indicatea dark element, providing an accurate reading.

In order to address the problem of varying the resolution two readingspots are provided of different resolutions and are read bycorrespondingly configured detectors wherein the detectors include morethan one channel, each channel corresponding to reading the indicia atthe different resolutions. Alternatively a single spot is provided,appropriate portions being read by the detector to provide reading atthe desired different resolution levels, and channels being providedcorresponding to the different resolutions. It will be seen that thecommon feature is that the system provides two data streams from the twodifferent resolution channels, for example to a single decoder which canbe common to the different alternatives.

For example, the embodiment of FIG. 3 has a single light source, lightemitting diode or “LED” 41, and a single photodetector, photodiode “PD”42. The LED 41 emits light to illuminate an area of the surface of theoptically encoded information, i.e. bar code symbol 20. The PD 42 senseslight reflected from bar code symbol 20 and produces an analog signalthe amplitude of which represents the amplitude of reflected light. Thereader scans the bar code symbol 20. If the reader is a wand, the usermanually passes the unit over the information such that the detectedreflected light varies in amplitude in correspondence with the light anddark regions of the information.

The analog signal from PD 42 is amplified, inverted and conditioned bytwo analog signal conditioning circuits 43 and 44. The signalconditioning circuits 43 and 44 are essentially identical and thusprovide two analog signals as output signals. One of these outputsignals goes to a first digitizer 45, the other to second digitizer 46.The digitizers 45 and 46 serve as edge detectors or wave shapercircuits, in a manner similar to digitizers used in prior art singlechannel type readers. In each of the digitizers 45 and 46, the thresholdvalue set by the digitizer determines what points of the analog signalrepresent bar edges. The digitizers 45 and 46, however, have differentthreshold values.

The pulse signals output from both of the digitizers 45 and 46 aresupplied as inputs to a programmed microprocessor type decoder 47.Signal conditioning circuit 43 and digitizer 45 from a first channelproviding a first data stream to the decoder 47. Signal conditioningcircuit 44 and digitizer 46 form a second channel providing a seconddata stream to the decoder 47. The threshold of the first digitizer 43is set relatively low that is, it is set to detect only substantialvariations in the amplitude of the analog signal so that digitizer 43will have a low resolution. The second digitizer 46 has a high thresholdset to detect minor amplitude variations and is sensitive.

Accordingly for either high resolution bar code symbols or lowresolution bar code symbols, one of the two channels will produce apulse signal output or data stream closely corresponding to the edges ofthe scanned bar code. The decoder 47 is a relatively standard unit, withthe exception that it has two inputs, instead of one, for the two datastreams for the two different resolution channels. The integrateddecoder 47 provides a digital data output, for example in ASCII format.

The embodiment of FIG. 4 produces two different channels optically. Thisembodiment includes two emitters and two detectors. A first LED 51 emitslight which illuminates a spot on the bar code 20. The light emittedfrom LED 51 is reflected back by the bar code and detected by a first PD52. A signal conditioning circuit 53 and first digitizer 54 provide alow resolution data stream for output as a pulse train signal to decoder47. A second LED 55 emits light which illuminates a second spot on thebar code 20. The light emitted from LED 55 is reflected back by the barcode and detected by a second PD 56. A second signal conditioningcircuit 57 and second digitizer 58 provide a low resolution data streamfor output as a pulse train signal to decoder 47.

As illustrated in FIG. 4, emitters and detectors are arranged so thatthe two spots are slightly spaced apart. This is called spatialmultiplexing. If it were desired to have the two spots concentric, otherforms of multiplexing could be used. For example, LED's 51 and 55 couldemit different wavelengths of light, and the associated PD's 52 and 56would be designed and/or have associated light filters so as to detectonly the light from the corresponding detector.

LED 51, PD 52, signal conditioning circuit 53 and digitizer 54 form thelow resolution channel. LED 55, PD 56, signal conditioning circuit 57and digitizer 58 form the high resolution channel. In this embodiment,the resolution and spot size of each channel is set by thecharacteristics of the LED, the PD, the associated optics, or anyapertures associated therewith. For example, optics can focus light fromeach LED to form a different size illuminated spot and/or at a differentdistance form the tip of the want. Alternatively, the size of the PD'scan vary or each can have a different aperture to establish a differentarea over which to average the reflected light. The digitizers 54 and 58may have the same threshold value, but preferably, the thresholds areset to correspond to the characteristics of the LED's, PD's and opticsof their respective channel.

A further variant shown in FIG. 5 uses two emitters and one detector.The output of the detector is multiplexed in synchronism with pulsing ofthe individual emitters to produce the two channels. The emitters and/orthe associated optics differ to provide the two different effectivesensing spots and the two different resolutions. The two spots can beclosely aligned or substantially concentric on the surface of theoptical information 20. The high and low resolution signals are timedivision multiplexed by the pulsing of the individual LED's.

The embodiment includes two LED's 61 and 62, but only one PD 63. Amultiplexer 64 alternately activates LED's 61 and 62. A multiplexer 65alternately provides the output of PD 63 to one of two sample and hold(S/H) circuits 66 and 67. The signal conditioning circuit 68 anddigitizer 70 provide the pulse signal and the low resolution channel tothe decoder 47. The signal conditioning circuit 69 and digitizer 71provide the pulse signal for the high resolution channel to the decoder47. A clock 72 provides the appropriate timing signals to the twomultiplexers 64 and 65 and the S/H circuits 66 and 67. Multiplexing canalso be performed by rapidly pulsing the two LED's at two differentfrequencies and performing frequency demodulation.

The LED 61 and its associated optics are designed to provide arelatively large illuminated spot, and the LED 62 and its associatedoptics are designed to provide a relatively small illuminated spot. Thesignal from clock 72 drives the multiplexer 64 to trigger LED 61 andmultiplexer 65 to provide the signal from PD 63 to S/H circuit 66. S/Hcircuit 66 holds a sample of the low resolution analog signal producedby the LED 61 and PD 63. The signal from clock 72 then drives themultiplexer 64 to trigger LED 62 and multiplexer 65 to provide thesignal from PD 63 to S/H circuit 67. S/H circuit 67 holds a sample ofthe high resolution analog signal produced by the LED 62 and PD 63. Asthis cycle repeats the S/H circuits 66 and 67 will successively holdsamples of the two different resolution analog signals.

The conditioning circuits and digitizers will then provide the twodiffering resolution data streams to the decoder 47. The signalconditioning circuits 68 and 69 and the digitizers 70 and 71 correspondclosely to those of the preceding embodiment.

The optics of the reader may comprise a system constructed by combiningtwo half-axicons each of which establishes a different depth of fieldand spot size so as to provide the two different resolutions with thetwo channels.

FIG. 6 is a simplified plan view of a photodetector having two activeareas, one surrounding the other, with Active area #1 shown as a shadedcentral circle and Active area #2 shown as a cross-hatched surroundingarea. The structure of the photodetector of this embodiment will bediscussed in more detail below with regard to FIGS. 7 and 8. The salientfeature of the photodetector is that it inherently forms a multi-channeldetection system. This multi-channel photodetector comprises a firstactive optical sensing area on a substrate and a second active opticalsensing area formed on the same substrate. The second optical sensingarea is located around said first optical sensing area. Each activeoptical sensing area, together with the underlying substrate forms, aphotodiode. The photodetector is used in the embodiment of the inventionshown in FIG. 9.

In the circuit of FIG. 9, the signal conditioning circuits and thedigitizers function in a manner similar to those in the embodiments ofFIGS. 4 and 5. The only different is that FIG. 9 shows the highresolution channel, including the second digitizer, as the upper channeland the first channel as the lower channel.

In this embodiment, there is one light emitting element and twophotodetectors, and the photodetectors comprise the active sensing areasof the unit shown in FIG. 6. In FIG. 9, D1 represents the sensor orphotodetector including Active area #1, the central active area. D2represents the sensor or photodetector including Active area #1, thesurrounding active area. The LED 131 emits light to illuminate theoptically encoded information. The photodetectors D1 and D2 receivelight reflected from the surface of the bar code 20.

Detector D1 will produce an analog signal which effectively representsthe average of reflected light received over the small Active area D1.This signal would be the same as if a photodiode of a small effectivearea were used, and the effective area established the spot size and/orresolution of the high resolution channel.

Detector D2 will produce an analog signal which effectively representsthe average of reflected light received over the larger surroundingActive area D1. The analog signals from D1 and D2 are summed by addingcircuit 132. The sum of the analog signals from D1 and D2 closelyapproximates the signal which a larger photodiode would produce, i.e. byaveraging received light over the total active area of area #1 plus area#2.

Signal conditioning circuit 133 receives the summed signal from adder132 and conditions it as discussed above. The signal from conditioningcircuit 133 is digitized by digitizer 134 to form the low resolutiondata stream. Detectors D1 and D2, adder 132, signal conditioning circuit133 and first digitizer 134 thus form the low resolution first channelin this embodiment. Signal conditioning circuit 137 receives the signalfrom D1 and conditions it as discussed above. The signal fromconditioning circuit 137 is digitized by digitizer 138 to form the highresolution data stream. Detector D1, signal conditioning circuit 137 andsecond digitizer 138 thus form the high resolution second channel inthis embodiment. Decoder 47 receives and processes the pulse signalsfrom digitizers 134 and 138 in the same manner as in the earlierdiscussed embodiments.

It would be a simple matter to increase the number of channels ofdifferent resolutions derived using the photodetector unit of FIG. 9 byadding additional surrounding areas and corresponding adders, signalconditioning circuits and digitizers. Alternatively, two active areascould be used and the two LED's in the sensing assembly pulsed, in amanner similar to that of the circuit of FIG. 5.

The photodetector unit of FIG. 9 would be fabricated using relativelystandard photodiode manufacturing technology. In particular, themanufacturing processes are similar to those used to build side by sidephotodiodes and quad four photodiode type devices. Inactive areas ordead zones between active devices typically range in size from 0.001 to0.005. Possible layouts for the photodetector unit of FIG. 9 appear inFIGS. 7 and 8.

The embodiment of FIG. 7 includes a substrate 141 on which the activeareas are formed. The first active area 142 is circular. The firstactive region 142 is formed by appropriately doping the circular region.A dead zone 143 surrounds the active area 142. The second active area144 forms a substantially circular ring around the first active area 142and dead zone 143. The second active region 144 is formed byappropriately doping the circular ring. The dead zone 143 separates andelectrically isolates active areas 142 and 144. A common lead 148 isattached to substrate 141 by bonding pad 147. Together with theunderlying substrate, each of the active regions 142 and 144 forms aphoto sensitive diode.

FIG. 7 illustrates the preferred form of connection to the first activearea 142. In this embodiment, the second active area 144 does not form acomplete ring around the first active area 142. A small inactive area149 forms an insulating passage through the ring formed by the secondactive area 144. A metal trace 145 formed on the small inactive area 149connects the first active area to a bonding pad. Current carryingconnection to the first active area can then be formed through thebonding pad and the metal trace 145. A metal trace 146 similarlyconnects second active area 144 to a bonding pad. Only a 2 mil wide paththrough the second active area 144 is lost to formation of the passage149 and trace 145.

FIG. 8 illustrates a second embodiment of the inventive photodetectorunit, having rectangular active areas using a bonding pad formeddirectly on each of the active areas. This embodiment includes asubstrate 151 on which the active areas are formed. The first activearea 152 is rectangular, and the first active region 152 is formed byappropriately doping the rectangular region. A rectangular dead zone 153is formed around the active area 152. The second rectangular active area154 completely encloses the first active area 152 and dead zone 153. Thesecond active region 154 is formed by appropriately doping the outerrectangular ring. The dead zone 153 separates and electrically isolatesactive areas 152 and 154. A common lead 158 is attached to substrate 151by bonding pad 157. Together with the underlying substrate, each of theactive regions 152 and 154 forms a photo sensitive diode.

FIG. 8 illustrates a second form of connection to the active areas. Inthis embodiment, a bonding pad is formed directly on each active area. Ametal lead 155 provides current carrying connection to the first activearea 152 through the bonding pad on that area, and a second metal lead156 provides a similar connection through the bonding pad on the secondactive area 154. In each active area a portion of the area is sacrificedto formation of the bonding pads. Also, the lead 155 will cast a shadowacross the second active area 154, as shown in FIG. 8.

Other active area type photodetector devices could be used. For example,it is contemplated that the photodetector unit could comprise areaswithin a two dimensional charge coupled device (CCD) array. The centralarea would comprise a number of pixel sensing units of the array, forexample, in the form of a 2×2 square sub-array. The surrounding activearea would comprise a number of pixel sensing units of the area aroundthe central active area, for example, forming a 2 pixel wide ring aroundthe square central active area. The signal from the central area wouldbe formed by shifting out the charge value of each pixel of the 2×2square sub-array and averaging the values over the number of pixels ofthe central area. The signal from the surrounding area could be formedin a similar manner, or the summation signal could be formed directly byaveraging values over both areas together.

In operation of a reader including two channels carrying data streams ofdiffering resolution, the data from both channels is read into adecoder. One of the data streams is chosen for decoding, and if thedecoder is successful an indicator such as a beep proceeds an output ofthe decoded data. If the decoder is unsuccessful the other data streamis decoded and, if successful the decoded data is output. If neitherdecode is successful then the signals can be merged, the decoderrecognising which portions of the data from each channel are acceptableand combining the acceptable portions to form a single final valid readresult.

Because of the provision of more than one channel at the photodetector,the signals of different resolutions can be separated out to respectivechannels allowing decoding of either the high resolution, the lowresolution, or a merged combined signal. Although specific embodimentsof light generating and light detecting elements have been describedabove it will be appreciated that variations of those embodiments may bearrived at which will cooperate successfully with the multi-channelphotodetector arrangement.

It will be appreciated that the decoder discussed above would alsoinclude appropriate software to allow auto-discrimination and capable ofdistinguishing between light reflected from the indicia to be read andundesirable accidental reflections from other objects in the vicinity.

The multi-channel detector arrangement may also be used in conjunctionwith devices capable of reading indicia at various distances from thereader using, for example, differently configured reflective surfacesand focusing lenses, whereby one channel is associated with the readingtaken at a first distance and a second channel is associated with thereading taken at a second distance. Once again the outputs from therespective channels can be decoded to establish which gives anacceptable read and may be merged, if necessary, in the manner discussedabove.

A further application for multi-channel optical detectors may be foundin methods of improving the signal to noise ratio when reading printedindicia such as bar code symbols. The portion of the analog signalcorresponding to the bar code information is much more regularlydistributed than the overlying electronic noise as the major noises suchas ambient noise, voltage or current amplifier noise at the front endare white noise. The invention thus provides improved signal to noiseratio by applying multi-channel time averaging to the incoming analogsignal in order to enhance the useful signal whilst depressing noise inorder that the signal to noise ratio can be significantly increased.

Operation of the system may be viewed in the block diagram of FIG. 10.The time averaging is carried out using a two channel processing unit70. An incident light beam 71 (analog signal) reflected from an indiciato be read is received by the unit 70 and transferred to a decoder 72via a first channel 73 and a second channel 74. The incoming signal isdigitized and sent to the decoder 72 directly via first channel 73. Thesignal is also digitized at unit 70 and sent to the decoder via thesecond channel 74 but a delay is induced (the second channel is called abuffer channel accordingly), as the analog signal is buffered in buffer75 associated with a second channel 74 with needed bit resolution.Generally a buffer 75 having 5K memory is sufficient. The bufferedsignal then joins at summing element 77 the conventional first channel73 signal which has proceeded from the next received incoming analogsignal. As a result the useful signal containing bar code information isenhanced whilst the noises are time averaged out. The signal having ahigher signal to noise ratio is then sent to the decoder 72 andtransferred from the decoder to appropriate processing means by via line76.

The operation of the system can be understood in more detail from FIG.11. When the arrangement shown in FIG. 10 is incorporated in a readersuch as that shown in FIG. 1, the bar code symbol being scanned is readrepeatedly, giving rise to analog signals S₁,S₂ at time intervals T1, T2etc. The analog signals S₁,S₂ may be in the form of pulse signals ofeffectively zeros and ones representing bars and spaces or vice versa.The profile of the pulse signal is, however, distorted by noise.Successive signals S₁,S₂ at time T1 and T2 at processing unit 70 areshown on channels 73 and 74. The signal S₁ on line 74 is delayed bybuffer 75 prior to arrival at summing element 77 such that it issynchronised with signal S₂ on line 73. The signals S₁,S₂ are thensummed at summing element 77 as a result of which the coarse pulseprofile relating to the bars and spaces is enhanced whereas the whitenoise is time averaged and suppressed.

According to another aspect of the invention further improvements to thereading of indicia and in particular bar code symbols printed in dotmatrix format are arrived at. Most of the time, failure to decode a dotmatrix symbol is caused by a split in a wide bar such that a void in thebar causes a narrow “white” element to appear in the middle of the bar(corresponding splitting of wide or narrow spaces have not, however,been found). Known digitizers are programmed when encountering a voidcausing a split in the wide element to record a white bar and thenrecord a black bar beginning at the edge of the void, finally switchingto a white bar when the true trailing edge of the bar is detected.Accordingly, data indicating where the real trailing edge of the bar islocated is retained but the situation could be improved if a distinctioncould be made between black to white transitions causes by defects andblack to white transitions caused by real bar edges. In that case thesymbol could be decoded even in the presence of false white elementsoccurring as a result of defects.

Research, however, shows that although the size of false white elementscaused by defects occurring within wide bars varies those elements havenot been found to have width any more than 60% of the width ofneighbouring true white elements. Generally the false elements are lessthan 50% of the size of true neighbouring white elements.

A solution to the problem is described with reference to the flow chartof FIG. 12. In effect the decoder is programmed to differentiate betweenwhite elements causes by defects and white elements caused by truespaces between bars and the symbol by comparing the size of a whiteelement with its neighbouring white elements. Referring to the flowchart, a trailing edge of a black element is first detected 80 (end ofbar) the width of the white element which may either be a true spacebetween bars or a defect is then scanned 81 (measure space). The decoderthen accesses a memory store in which it has stored information relatingto the width of white elements in previous steps. In particular thememory stores information relating to the width of neighbouring whiteelements. The width of the current white element is then compared withthe stored width 82 and if the width of the current element is less than60% [83] of the stored width value then it is identified as a defect 84.In that case the decoder adds the width of the false white element tothe width of the black bar elements on either side 85 (add to bar width)and the true bar width is thus determined. If at step 83 the whiteelement has a width greater than 60% of the stored width then it isidentified as a true space 87 (determined space width) having the widthdetermined in step 81. In either case, operation then continues asnormal.

Generally the memory need only store information as to the width of theprevious character which, if decoded and identified as a true space willact as a sufficient template for comparison with the subsequent spacewidth.

Generally to assist in identifying the width of true white elements astart character of predetermined width is provided which is recognisedby the decoder as a model true width and can be used as a basis forlater comparisons. Defects may occur even in the start character,however, in which case if an initial value of a valid space (startcharacter) cannot be found owing to defects in the printing of the wideelements in the start character, decoding may be carried out by scanningin the opposite direction, that is from the other end of the symbol.

In order to enhance the process, the decoder may be pre-programmed withinformation relating to the true space widths of the symbols to bedecoded although this must, of course, be normalised with regard to thedistance of the reader from the symbol to be read. Alternatively,suitable algorithms may be introduced to assist the decoder in initiallysetting values for valid space widths, for example the decoder mayestablish a table of detected space widths and deduce the narrowestvalid space width based on the 40% size gap between the narrowest validspace and the widest invalid space. Although, generally such additionalsteps would not be required it will be seen that in exceptionalcircumstances where more than one false space is found in a single barthis would not present a problem to the modified type decoder.

The bar code may be detected by a field of view CCD array in which casethe relative physical sizes of the bar spaces can be compared.Alternatively the bar code can be scanned by a laser or light spot. Inthat case, as the scanning will generally take place at a constant rate,the relative sizes of bars and spaces can be determined from the periodof time detected between the leading and trailing edges of bars orspaces. The process discussed above can be further modified in variousways. For example any space more than approximately 50 microsecondsscanning duration can generally be assumed to be a reject because inreality no elements are that small. Small defect rejection can becombined with ASCII stitching (combining of partial scans to form acomplete scan), half-block decode (combining of separately acquiredhalves of the symbol) or distance decode. It may be found in scanning abatch of symbols printed during the same process that differentrejection thresholds are appropriate, for example 40% or 50% ofneighbouring elements. The algorithm within a decoder may be arranged tovary the rejection threshold based on a record of the compared ratios offalse and true white elements.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can readily adapt to variousapplications without omitting features that, from the standpoint ofprior art, fairly constitute essential characteristics of the generic orspecific aspects of the invention and, therefore, such adaptions shouldand are intended to be compounded within the meaning and range ofequivalents of the following claims.

1. A method of processing a bar code symbol composed of elements of lowand high reflectivity and compensating for a discovered defect thereincomprising scanning the bar code symbol with a light beam, detecting thereflected light beam and providing a digitized signal carrying the barcode information to a processor, wherein the processor compares thewidth of elements of high reflectivity with a predetermined minimumwidth for elements of high reflectivity in a bar code symbol, identifiesthe element as a defect if its width is less than the predeterminedminimum width, and if the high reflectivity element is identified as adefect the digitized signal is corrected by representing the element asthough is has low reflectivity in a corrected digitized signal that isthen used in decoding the symbol thereby ignoring the effect of the highreflectivity element defect during decoding of the bar code symbol.
 2. Amethod as claimed in claim 1 in which, if the high reflectivity elementis identified as a defect, the width of the defect element is added tothe width of the low reflectivity elements on either side of thehigh-reflectivity element to form a single low-reflectivity elementhaving a width generally equal to the combined width of the defectelement and the low-reflectivity elements on either side of the defectelement.
 3. The method of claim 2 wherein the high-reflectivity elementsare generally white and the low reflectivity elements are generallyblack.
 4. The method of claim 1 wherein the minimum width is determinedfrom the width of at least one non-defective element of highreflectivity in the bar code symbol.
 5. The method of claim 4 whereinthe at least one non-defective element of high reflectivity is thepreviously scanned element of high reflectivity.
 6. The method of claim5 wherein the percentage is 60%.
 7. The method of claim 4 wherein theminimum width is a percentage of the width of the non-defective elementof high reflectivity.